The present invention relates to a sliding member of graphite-metal borides which is useful as the member for the parts where friction is caused by sliding in bearings, cylinders, slide valves, etc., and more in detail, the present invention relates to a novel sliding member showing favorable specific properties to friction and abrasion, particularly at high temperatures, which is favorably suitable fitted for use under such conditions where the use of a fluid lubricant such as lubricating oils and greases is difficult, i.e., the so-called dry-friction conditions.
In recent years, with the development of mechatronics, the demands for the sliding members, which are excellent in frictional and abrasive properties, particularly in a high temperature range has been raised. For instance, since a fluid lubricant such as lubricating oils and greases cannot be used between the mutually contacting surfaces of the materials of the sliding part at a high temperature in a range of 200.degree. to 500.degree. C., a solid lubricant such as graphite and molybdenum disulfide is usually used. However, such a solid lubricant have no fluidity nor wettability as are generally seen in a fluid lubricant and accordingly, the solid lubricant cannot be applied by the means such as painting, impregnating, pouring, etc.
Namely, in the case of using a solid lubricant, it must be applied by the means such that (1) the friction surface is covered with a solid lubricant, (2) the sliding member per se are prepared of a solid lubricant, (3) the solid lubricant is used after being admixed with another material, etc. Since the lubricating effect of the solid lubricant used in any one of the above-mentioned means is exhibited at the first time after a part of the solid lubricant thus applied on the sliding, friction surface is transferred to the surface of the mating material by forming a film of a desired thickness thereon.
Accordingly, the frictional and abrasion properties of the thus applied solid lubricant is affected largely by film-forming property of the solid lubricant on the surface of the mating material.
As the solid lubricant for satisfying such a requirements, graphite which is excellent in self-lubricating property and thermal conductivity, non-melting property, small in thermal expansion and in addition, excellent in heat-resistance has been most frequently used.
The graphite used as sliding member is classified into natural graphite and artificial graphite in general, and in the field of which a solid lubricant itself is used as the sliding members, artificial graphite which is a little poor in the point of frictional and abrasive properties, however, by which shaped articles of a high strength are available has been generally used.
As the method for obtaining the shaped articles of a high strength from artificial graphite, a method comprising the steps of (1) molding an aggregate, for instance, powdery coke while using a binder such as coal-tar pitch which is carbonized or graphitized by heating or calcining, (2) calcining the thus molded material at a temperature of 700.degree. to 1000.degree. C., (3) impregnating the thus calcined material with an impregnating agent such as pitch, and (4) re-calcining the thus impregnated material and then thermally treating at a temperature of 2500 to 3000.degree. C., thereby graphitizing the thus impregnated material has been hitherto adopted. In addition, in order to obtain a graphite shaped-articles of a higher strength, a method of impregnating the thus obtained graphite shaped-articles with a resin or a metal has been adopted.
Although the graphite shaped-articles of a high strength can be used as the sliding members, the frictional and abrasive properties and particularly, the film-forming property on the mating surface are still insufficient, and as a result, such a sliding members have demerits of high in friction coefficient and large in amount of abrasion. In addition, there is another demerit that it takes a very long term for graphitizing the powdery coke (generally, it takes 2 to 3 months for graphitizing the coke).
Formerly, the present inventors have proposed a process for producing a high-density graphite-boron carbide composite material, comprising adding boron carbide as a sintering-accelerator to powdery amorphous carbon or powdery artificial graphite and sintering the thus prepared mixture at a temperature of higher than 2000.degree. C. under a pressure of higher than 200 kg/cm.sup.2 (refer to Japanese Patent Publication No. 56-37192 (1981)).
Further, the present inventors have found that graphitization of powdery coke is accelerated by sintering a mixture of powdery coke and powdery boride of a metal of IVa group, Va group or VIa group of the periodic table for 30 min at a temperature of 2100.degree. C. under a pressure of 200 kg/cm.sup.2 (refer to "YOGYO KYOKAISHI" (Journal of the Ceramic Soc. of Japan), Vol. 8, No. 8, pp 416-422, 1979).
In addition, as a result of studying for solving the aforementioned defect of the sliding members, the present inventors have found a high temperature-resistant and abrasion-resistant sliding member of graphite-boron carbides produced by the steps of (1) mixing and stirring 65 to 95% by weight of powdery carbon of an average particle size of 10 to 150 .mu.m and 5 to 35% by weight of powdery boron carbide of an average particle size of 0.5 to 2.5 .mu.m until the powdery boron carbide is uniformly dispersed in the powdery carbon and (2) sintering the thus prepared mixture at a temperature between 2020.degree. C. and the transition point of the structure of the thus formed mixture (refer to U.S. patent application Ser. No. 786,086, filed July 17, 1985).
For providing a novel sliding members, which are more excellent in frictional and abrasional properties at high temperature and can be used under the dry-friction conditions in which a liquid lubricant such as lubricating oil can be hardly used, the present inventors have further studied on the method for producing a high-density graphite composite material from a mixture prepared by adding and mixing a specific amount of a powdery boride to powdery coke, i.e., the method for producing a high-density graphite composite material by the steps of (1) adding and mixing a specific amount of powdery borides to powdery amorphous carbon such as powdery coke, (2) uniformly blending the thus obtained mixture and (3) sintering the thus obtained material under a pressure, and as a result of the present inventors' studies, it has been found that graphite-metal borides produced by sintering a mixture of (a) 70 to 93% by weight of powdery amorphous carbon, (b) 5 to 20% by weight of at least one powdery boride of a metal of IVa group, Va group or VIa group of the periodic table and (c) 2 to 10% by weight of powdery natural graphite, powdery Kish graphite or a mixture thereof, is the material for sliding members having more excellent specific properties, and based on the present inventors' finding, the present invention has been attained.
Namely, the object of the present invention is to be provided with sliding members which solve the demerits of transferring property of the solid lubricating film to the mating surface and the film-forming property of the material for sliding member as well as the frictional and abrasional property thereof at high temperature.